Hitherto, cemented carbide cutting tools made of a WC-Co alloy or a WC-Co alloy to which a carbonitride of Ti and/or Ta, Nb, or the like has been added have been used for cutting of general steel or castings. During cutting, the cutting edges of the cutting tools are heated to high temperatures of 800° C. or more. The cemented carbide cutting tools therefore tend to cause plastic deformation because of heat generated by cutting. Thus, the flank wear tends to increase.
Thus, surface-coated cutting tools that include a base material (substrate) made of cemented carbide covered with a monolayer of carbide, nitride, or carbonitride of a group IVa metal (such as TiC, TiN, or TiCN) or a hard ceramic, such as Al2O3, or with a composite layer of the hard ceramics have been used so as to improve the cutting characteristics of the cutting tools at high temperatures. The covering layer is formed by a chemical vapor deposition method or a physical vapor deposition method, such as an ion plating method or an ion sputtering method.
Among the covering layers formed by these methods, a covering layer formed by a chemical vapor deposition method particularly has excellent adhesion to the cemented carbide base material and very high abrasion resistance. In response to the recent demand for high speed and high efficiency cutting, the thickness of the covering layer is increasing. Thus, the adhesion between the cemented carbide base material and the covering layer is important.
In the formation of the covering layer by a chemical vapor deposition method, the temperature of the covering layer is as high as approximately 1000° C. When cooled to room temperature after the formation of the covering layer, therefore, the covering layer has tensile stress caused by a difference in thermal expansion coefficient between the cemented carbide base material and the covering layer. The tensile stress propagates a crack occurring on the surface of the covering layer during cutting and causes falling off or chipping of the covering layer. More specifically, the cemented carbide base material has a thermal expansion coefficient of approximately 5.1×10−6 K−1, whereas the covering layer has a thermal expansion coefficient of approximately 9.2×10−6 K−1 for TiN, approximately 7.6×10−6 K−1 for TiC, or approximately 8.5×10−6 K−1 for Al2O3.
The thicknesses of the covering layers of commonly used surface-coated cutting tools are in the range of several micrometers to around ten micrometers. This is because although the abrasion resistance of the covering layer increases with the thickness, an increase in the thickness of the covering layer results in a higher possibility of damaging the tool and lower defect resistance of the surface-coated cutting tool.
Thus, various techniques have been proposed to improve the characteristics of the covering layer. For example, Japanese Unexamined Patent Application Publication No. 07-216549 (PTL 1) describes a technique for increasing the cutting life of a cutting tool by eliminating cracks in cooling after the formation of an alumina layer.
In accordance with Japanese Unexamined Patent Application Publication No. 05-057507 (PTL 2), the outermost layer made of TiN or TiCN is formed on the surface of a tool base material composed of a hard material by a chemical vapor deposition method, an Al2O3 film is formed as an inner layer adjacent to the outermost layer, and only a cutting edge portion of the tool is polished. In accordance with this technique for extending the life of the tool, the Al2O3 film thus exposed to the atmosphere can improve the welding resistance of chips and impact resistance during cutting.
CITATION LIST
Patent Literature
PTL 1: Japanese Unexamined Patent Application Publication No. 07-216549
PTL 2: Japanese Unexamined Patent Application Publication No. 05-057507